The present invention relates to a light wood-containing material having an average density in the range from 200 to 600 kg/m3, comprising, based in each case on the wood-containing material:    A) from 30 to 95% by weight of wood particles;    B) from 1 to 15% by weight of a filler having a bulk density in the range from 10 to 100 kg/m3, selected from the group consisting of foamable plastic particles and already foamed plastic particles;    C) from 3 to 50% by weight of a binder comprising an aminoplast resin and an organic isocyanate having at least two isocyanate groups and, if appropriate,    D) additives.
The present invention furthermore relates to a multilayer wood-base material comprising the wood-containing material according to the invention, a process for the production of light wood-containing materials, a process for the production of a multilayer wood-base material, the use of the light wood-containing material according to the invention and of the multilayer wood-base material according to the invention, the use of a binder comprising an aminoplast resin and an organic isocyanate having at least two isocyanate groups, the molar formaldehyde: —NH2 group ratio being in the range from 0.3 to 1.0, for the production of wood-containing materials or multilayer wood-base materials having a density in the range from 200 to 600 kg/m3.
Wood-base materials, in particular multilayer wood-base materials, are an economical and resource-protecting alternative to solid wood and have become very important in particular in furniture construction, in laminate floors and as construction materials. Starting materials used are wood particles of different thickness, for example wood chips or wood fibers from various timbers. Such wood particles are usually pressed with natural and/or synthetic binders and, if appropriate, with addition of further additives to give sheet-like or strand-like wood-base materials.
In order to achieve good mechanical properties of the wood-base materials, the latter are produced with a density of about 650 kg/m3 or more. Wood-base materials having this density or the corresponding parts, such as furniture, are often too heavy for users, in particular private consumers.
The industrial demand for light wood-base materials has therefore increased in recent years, in particular since take-away furniture has grown in popularity. Furthermore, the increasing oil price which leads to a continuous increase in, for example, the transport costs is giving rise to greater interest in light wood-base materials.
In summary, light wood-base materials are of major importance for the following reasons:
Light wood-base materials lead to simpler handling of the products by the end customers, for example during packing, transporting, unpacking or assembly of the items of furniture.
Light wood-base materials lead to lower transport and packaging costs; furthermore, material costs can be cut in the production of light wood-base materials.
For example, when used in means of transport, light wood-base materials can lead to lower energy consumption of these means of transport. Furthermore, with the use of light wood-base materials, for example, material-consumptive decorative parts, such as thicker worktops and side panels which are currently in fashion in kitchens, can be made available more economically.
Against this background and against the background of an increasing regulation for further reduction of formaldehyde emissions from wood-base materials, there is a wish for providing light wood-base materials having lower formaldehyde emission but, as in the past, good performance characteristics and processing properties.
The prior art contains a variety of proposals for reducing the density of wood-base materials.
For example, tubular particle boards and honeycomb boards may be mentioned as light wood-base materials which are obtainable by constructional measures. Owing to their particular properties, tubular particle boards are used mainly as inner layer in the production of doors.
For example, the excessively low screw withdrawal resistance, the complicated fixing of fittings and the difficulties in edging are disadvantageous in the case of the honeycomb board.
Furthermore, the prior art contains proposals for reducing the density of the wood-base materials by additions to the glue or to the wood particles.
CH 370229 describes light moldings which are simultaneously pressure-resistant and consist of wood particles or wood fibers, a binder and a porous plastic serving as filler. For the production of the moldings, the wood particles or fibers are mixed with binder and foamable or partly foamable plastics and the mixture obtained is pressed at elevated temperature. Binders which may be used are all customary binders suitable for the gluing of wood, such as, for example, urea-formaldehyde resins. Suitable fillers are foamable or already foamed plastic particles, preferably expandable thermoplastics, such as styrene polymers. The boards described in the examples have a density of from 220 kg/m3 to 430 kg/m3 and an average flexural strength of from 3.6 N/mm2 to 17.7 N/mm2 at a thickness of from 18 to 21 mm. The transverse tensile strengths are not stated in the examples. Regarding formaldehyde emission or a combination of aminoplasts and isocyanates in the binder, CH 370229 makes no statement.
WO 02/38676 describes a process for the production of light products, in which from 5 to 40% by weight of foamable or already foamed polystyrene having a particle size of less than 1 mm, from 60 to 95% by weight of lignocellulose-containing material and binder are mixed and are pressed at elevated temperature and elevated pressure to give the finished product. The customary binders are mentioned, inter alia MDI. Regarding formaldehyde emission or a combination of aminoplasts and isocyanates in the binder, WO 02/38676 makes no statement.
US 2005/0019548 describes light OSB boards using fillers having a low density. Polymeric binders, for example diphenylmethane 4,4-diisocyanate resin, are described as binders. Glass, ceramic, perlite or polymeric materials are described as fillers. The polymeric material is used in an amount of from 0.8 to 20% by weight, based on the OSB board. In the examples, the material Dualite, which consists of polypropylene, polyvinylidene chloride or polyacrylonitrile, is used as polymeric material. A weight reduction of 5% is described. In the examples, OSB boards having a density of from 607 to 677 kg/m3 and a transverse tensile strength of from 0.31 to 0.59 N/mm2 are described. Regarding formaldehyde emission or a combination of aminoplasts and isocyanates in the binder, US 2005/0019548 makes no statement.
JP 06031708 describes light wood-base materials, a mixture of 100 parts by weight of wood particles and from 5 to 30 parts by weight of particles of synthetic resin foam being used for the middle layer of a three-layer particle board, these resin particles having a specific gravity of not more than 0.3 g/cm3 and a compressive strength of at least 30 kg/cm2. Furthermore, it is stated that the specific density of the wood particles should not exceed a value of 0.5 g/cm3. According to JP 06031708, the binders are not subject to any restrictions and it is possible to use the customary ones, inter alia polyfunctional isocyanates. Regarding formaldehyde emission or a combination of aminoplasts and isocyanates in the binder, JP 06031708 makes no statement.
EP 0 025 245 B describes a process for the production of particle boards comprising a binder of polyisocyanate and aminoplast glue, the aminoplast glue being prepared with the use of from 0.25 to 0.625 mol of formaldehyde per mole equivalent of amino groups. EP 0 025 245 B does not disclose fillers or the density of the particle boards.
In summary, the disadvantage of the prior art is that the light (wood-base) materials described have a mechanical strength which is too low, such as, for example, a screw withdrawal resistance which is too low, for furniture production.
A mechanical strength which is too low can, for example, lead to breaking or tearing of the structural elements. Furthermore, these structural elements tend to exhibit additional chipping off of further wood material during drilling or sawing. In the case of these materials, the fixing of fittings is difficult.
Furthermore, the edging properties, i.e. the application and adhesion of edge materials, for example to particle boards, are still unsatisfactory in the prior art.
With regard to the formaldehyde emissions, too, there remains room for improvements in the case of the wood-base materials of the prior art.